1. Field of the Invention
This invention relates to a system and method of attitude control for flight vehicles including missiles, kill vehicles and space craft.
2. Description of the Related Art
Attitude control systems are used to maneuver flight vehicles such as a missile, kill vehicle (KV) or space craft. One approach is to control moveable fins or other airfoils to produce the force vector required to pitch and yaw the flight vehicle. This approach is relatively inexpensive but is not capable of maneuvering the vehicle with both speed and precision, has moving parts that reduce reliability and will not work in space. Another approach is to use active thrusters. This approach is considerably more expensive and has its own reliability concerns due to the complexity of the thrusters and closed-loop control systems. Active thrusters can produce larger force vectors to reduce the response time of any maneuver.
As shown in FIG. 1, a missile 10 is oriented with its body axis 12 along the x-axis. The missile's rocket motor 14 produces a thrust vector 16 that ordinarily imparts a force on the missile along its body axis. An articulated nozzle 18 turns thrust vector 16 to impart a pitch (rotation about the y-axis) or yaw (rotation about the z-ax) moment to the missile. An IMU 20 measures the attitude of the missile and provides active feedback control to articulated nozzle 18. This is known as “Thrust Vector Control” and is typically used with tactical missiles during flight to provide relatively gross and slow attitude control. A related approach is to fix nozzle 18 but place turning vanes inside the nozzle to turn the thrust vector. This is known as “Jet Vane Control”. Both techniques require moving parts, are relatively expensive solutions and are generally incapable of executing tight turning radius maneuvers.
Another approach is to use a combination of roll-control thrusters and pitch-over thrusters as illustrated in FIGS. 2a-2c and 3a-3c. A pair of roll-control thrusters 30 and 32 are mounted on opposite sides of a missile 34 so that their thrust vectors produce a rotational moment that “rolls” the missile around its body axis in either direction. A pair of pitch-over thrusters 36 and 38 are mounted on opposite sides of missile 34 in the pitch plane and offset from the missile's center of gravity so that their thrust vectors produce a rotational moment that “pitches” the missile. Because the missile's moment of inertia to pitch is much greater than its moment of inertia to roll, the pitch thrusters are generally capable of producing substantially more thrust than are the roll-control thrusters. Both types of thrusters are typically fixed amplitude and variable pulse-width to control the total applied force. Although the roll-control and pitch-over thrusters are shown on the missile orthogonal to each other, they can be aligned or have any desired orientation with respect to each other because they act independently.
To maneuver missile 34 from its current heading to an attitude 40, the roll-control thrusters 30 and 32 and than the pitch-over thrusters 36 and 38 are fired in sequence. Because the “roll” and “pitch” maneuvers are performed sequentially, any error in the “roll” maneuver will induce a large error in the “pitch”. Consequently, active closed-loop control is used for both maneuvers. As shown in FIGS. 2a and 3, roll-control thruster 30 is fired producing thrust vector 42 that rolls the missile in a counter-clockwise direction. In an active closed-loop control system, the IMU will constantly measure the roll angle and feed it back to the attitude controller, which in turn will either continue to fire thruster 30 to continue rolling the missile or, if the missile has rolled to far, fire thruster 32 produces thrust vector 44 to roll it back. Once the roll maneuver has stabilized so that the plane of pitch-over thrusters 36 and 38 is aligned with desired attitude 40, pitch-over thruster 36 is fired producing a thrust vector 46 that rotates the missile around its center of gravity in the pitch plane. The attitude controller actively controls pitch-over thrusters 36 and 38, which produce thrust vectors 46 and 48, to stabilize the orientation of the missile along attitude 40. Although this approach is widely used for attitude control in large, expensive missile systems, the attitude control system is expensive and less reliable on account of the pulse-width modulated thrusters and active control system and relatively slow on account of the sequential “roll” and “pitch” maneuvers.
In modern weapons systems and space craft demands are being placed on the attitude control systems to be able to perform attitude maneuvers very quickly and precisely with high reliability and at low cost. Smaller and lower cost flight vehicles are being planned for deployment in much higher volumes that are placing higher demands on performance at lower costs. The known approaches for attitude control cannot meet the cost-performance requirements of these systems.